The present invention relates to improvements in processes for the preparation of sodium percarbonate, and more specifically, it relates to processes for the preparation of sodium percarbonate having the formula 2Na.sub.2 CO.sub.3.3H.sub.2 O.sub.2 by the action of aqueous hydrogen peroxide on hydrated sodium carbonate.
U.S. Pat. No. 4,022,874 describes processes for the preparation of sodium percarbonate by the action of aqueous hydrogen peroxide solutions containing from 60% to 80% hydrogen peroxide on particular sodium carbonates. These prior art processes are characterized by the fact that the sodium carbonate feed material is selected from sodium carbonate monohydrate and hydrated sodium carbonates containing from 75 to 90% sodium carbonate.
It is desirable, when the sodium percarbonate is to be used as a component of detergent compositions, that the finished material have, in addition to good resistance to shock and attrition, an apparent density of from about 0.6 to about 1 g/cm.sup.3 and a mean diameter of from about 210 to 610 microns. According to the procedure for the preparation of such percarbonates, disclosed in the aforesaid Patent, the sodium carbonate feed should have an apparent density of from about 0.450 to 0.750 g/cm.sup.3 and a mean diameter of from about 200 to about 450 microns. The fact that the feed sodium carbonate has an apparent density in the range of 0.450 to 0.750 g/cm.sup.3 is somewhat of an inconvenience in the preparation of the sodium carbonate monohydrate.
An economical method for preparing the monohydrate is, in effect, to carbonate 48% commercial soda with purified combustion gas containing 12% carbon dioxide by volume. This carbonation method is on the one hand advantageous in obtaining a good recovery of carbon dioxide during the course of the carbonation operation because it reduces the size of the purification apparatus for the combustion gas and that of the compressor utilized to introduce the gas into the soda carbonation apparatus. On the other hand, the carbonation method simultaneously causes precipitation of sodium carbonate monohydrate under the usual conditions of temperature and pressure and permits an advantage in the process heat requirements by eliminating surplus water.
One of the conditions for obtaining a high recovery yield of carbon dioxide is that the carbonate solutions contain an excess of free soda. However, under these conditions the monohydrated carbonate which precipitates is in the form of very compact crystals which reach or surpass a density of 1.2 g/cm.sup.3 and which are only very slightly reactive with hydrogen peroxide.